Graphene is a semi-metallic material having an atomic layer in which carbon atoms are hexagonally arranged and connected by sp2 bonding in two dimensions. Recently, a single-layered graphene sheet was stripped from graphite, and the characteristics thereof were evaluated. It was discovered from the evaluation results that the electron mobility of the single-layered graphene sheet was 50,000 cm2/V·s or more, and that an electron moves at light velocity as if it does not have mass. Further, graphene is characterized in that it has structural and chemical stability and high thermal conductivity. Moreover, it is easy to form a one-dimensional or two-dimensional nanopattern on graphene because it is composed of only carbon which is a light element.
Most of all, graphene, which is a cheap material, has high price competitiveness compared to conventional nanomaterials. Thanks to such electrical, structural, chemical and economical characteristics, it is predicted that graphene will be used in silicon-based semiconductor technologies and will be used in manufacturing electrodes for electric and electronic devices and energy devices. Particularly, thanks to excellent mechanical properties, it is expected that graphene can be practically used in the field of flexible electronic devices.
In spite of excellent properties of graphene, a method of synthesizing high-quality graphene in large amounts and a method of dispersing graphene at high concentration have not yet been developed, and thus research into practically applicable technologies has been very limited.
For example, a graphene film, which is obtained by chemically and mechanically processing graphite crystals using a conventional wet processing method such as Hummers method (W. Hummers and one person, J. Am. Chem. Soc., 80, 1339, 1958), Brodie method (B. C. Brodie, Ann. Chim. Phys., 59, 466-472, 1860) or Staudenmaier method (L. Staudenmaier, Ber. Dtsch. Chem. Ges., 31, 1481-1499, 1898), can be commercially applied, and can be directly used in the field of electronic printing, such as spray coating, dip coating, spin coating, screen printing, inkjet printing, pad printing, knife coating, kiss coating or gravure coating. However, it is difficult to prepare a paste using a high-quality and high-concentration graphene dispersed solution.
The difficulty in preparing a paste is attributable to an aggregation phenomenon occurring in a solvent due to the fact that hydrophilic graphene is converted into hydrophobic graphene during a process of reducing a hydrophilic graphene oxide dispersed solution. Particularly, in the case of graphene oxide having low defectiveness and high purity, the aggregation phenomenon in a dispersion solvent becomes more serious after the conversion of graphene oxide into reduced graphene oxide.
Generally, a method of preparing graphene includes the steps of: forming graphite oxide and dispersing the graphite oxide in an aqueous solution; and reducing graphene oxide. Here, the graphene oxide is formed by a stripping process using an ultrasonicator.
However, since various defects are formed during acid treatment and ultrasonication, and oxygen-containing functional groups exist on the surface of graphene, graphene oxide has semi-conductive properties. In order to solve this problem, reduced graphene oxide is prepared by chemical and thermal reduction. In this case, the property of the reduced graphene oxide is changed from hydrophilicity to hydrophobicity, and a deposit is formed due to the aggregation thereof in a solvent to inhibit the formation of a high-concentration dispersion solution.
Recently, in order to apply a printing process for flexible electric/electronic devices, research into high-concentration and high-crystallinity reduced graphene oxide has actively been conducted. Particularly, professor Rod Rouoff's study group of Texas Austin University in the U.S.A. is doing research into the formation of a graphene-dispersed solution using a wet process.
As such, a polymer-based surfactant is used for the purpose of the stable dispersion of reduced graphene oxide (Stankovich et al., J. Mater. Chem. 2006, 16, 155). However, the usage of the polymer-based surfactant may lead to the deterioration of physical and chemical characteristics of nanomaterials. Further, when an alkali solvent is used, the interaction of a cation and an oxygen function group of graphene oxide occurs, and thus reduced graphene oxide can be stably dispersed in a solvent by Coulomb's repulsion between cations (Park et al., Nano Lett. 2009, 9, 1593.). However, in the case of low-defectiveness/high-purity graphene oxide, a hexagonal sp2 region is relatively widely exposed, and thus the interaction thereof with an oxygen function group is weak. Therefore, a new dispersion concept is required.